1. Field of the Invention
The present invention relates to a synthetic resin composition. More particularly, it relates to a synthetic resin composition comprising a hydrogenated block-copolymer of an aromatic vinyl compound and a conjugated diene compound, and polyolefinic compounds which is non-toxic and superior in softness, transparency, high-temperature strength and molding-characteristic.
2. Brief Description of the Prior Art
Hitherto, various synthetic resin compositions for molding have been proposed and practically used to obtain transparent and soft mold goods in the field of food packing material, medical appliances and the like. However, these known synthetic resin compositions all have such problems as described below.
That is, mold goods prepared from ethylene/vinyl acetate copolymer, ethylene/acrylate copolymer, 1,2-polybutadiene, metal salt of ethylene/acrylic acid or methacrylic acid copolymer, or the like are favorable in softness and transparency, but have problems that their tensile strengh is only 200 kg/cm.sup.2 or less and their thermal resistance is so insufficient that they cannot keep their shape at a temperature of 100.degree. C. or lower because of softening. Therefore, these mold goods have a defect that they cannot be sterilized by heating at a high temperature. Although thermoplastic polyurethane and thermoplastic polyester are superior in softness, tensile strength and thermal resistance, their transparency is not necessarily sufficient and moreover they have such defect that they are expensive and sanitary problems arise if their monomer remains. Elastomers of silicone series also are very expensive, though they are superior in softness, tensile strength and thermal resistance and have good transparency.
Further, it is known that mold goods based on such thermoplastic elastomers as styrene/butadiene block copolymer, styrene/butadiene/styrene block copolymer, styrene/isoprene/styrene block copolymer and the like are superior in softness and transparency and have a tensile strength of 200 kg/cm.sup.2 or more. However, they also have a problem that their thermal resistance is insufficient and so they cannot keep their shape even at a temperature of 100.degree. C. or lower because of softening.
To improve said copolymers with regard to this problem, hydrogenated block copolymers of styrene/butadiene/styrene block copolymer or styrene/isoprene/butadiene copolymer have been proposed and put on the market recently. These hydrogenated block copolymers are superior in softness, transparency and tensile strength. However, their thermal resistance is still insufficient though it is considerably improved as compared with that of unhydrogenated ones. Further, they are abnormal in their fluid-characteristic in the molten state and inferior in their molding-characteristic, especially, extrusion molding-characteristic. Therefore, in the field of engineering plastics, they are used mainly for the purpose of improving other resins by adding thereto in a small amount (cf. U.S. Pat. Nos. 4,081,424, 4,080,357, 4,111,896, 4,080,356, 4,090,996, 4,111,895, 4,088,627 and 4,088,626).
One exceptional example of using said hydrogenated block copolymers as the main component is their composition with polypropylene and ethylene-vinylacetate copolymer (cf. U.S. Pat. No. 4,140,162), which however is insufficient in softness at a low temperature, strength at a high temperature and the like.
Thermoplastic elastomers of ethylene/propylene copolymer series or those based on ethylene/propylene copolymer and polypropylene are soft and their thermal resistance is relatively good. However, they are considerably inferior in transparency and tensile strength.
In contrast, molding goods prepared from soft polyvinyl chloride are superior in softness and transparency, have a tensile strength of such degree as 200 kg/cm.sup.2, and keep their shape even at a high temperature of such degree as 120.degree. C. while maintaining sufficient strength. Therefore, they can be sterilized by heating at a high temperature and are widely used not only in the field of general packing industry but also for food packing, medical appliances and the like. However, soft polyvinyl chloride usually contains a large quantity of plasticizer and is hardened by slow release of the plasticizer. Thus, it is feared that sanitary problems arise by transfer of the plasticizer into the contents in contact with packing bags, tubes of the like made of such soft polyvinyl chloride. Further, there was a fear of sanitary problems arising unless the amount of vinyl chloride monomer remaining in the mold goods could extremely be diminished.
Thus there has been an earnest desire for synthetic resins for molding, which are sufficient in softness, strength, thermal resistance and transparency, which do not contain such highly toxic plasticizer and monomer as the soft polyvinyl chloride does, and which have good molding-characteristic.
The present invention has been made to solve the above-mentioned subject that could hardly be solved by previously known resin compositions.
Thus, it has been found that substantially non-toxic synthetic resin compositions which are sufficient in softness, strength, thermal resistance and transparency and have good molding-characteristic can be obtained by using a hydrogenated block copolymer which has been hitherto used in a relatively small amount for improving other resins, as the main component of the compositions, in combination with a specific resin of polypropylene series and also with a specific ethylene/acrylate copolymer.